1. Field of the Invention
The invention relates to methods and apparatus for pulse transit time-based distance measurement
2. Description of the Related Art
A pulse transit time-based approach is typical among the methods for distance measurement. In these methods, a variety of terms such as “transit pulse”, “wave pulse, “wave train” or “pulse burst” define the same—a pulse or group of pulses transferable by an oscillating agent (a waveform) from the pulse generating device toward a target; said pulse burst then reflected at the target presumably back to the pulse generating device. Any pulse transit time-based method for distance measurement requires knowledge of the time elapsed between the transmission of the wave pulse and receiving the wave pulse reflected at the target: pulse transit time (PTT). The oscillating agent could be of a different physical origin including but not limited to electromagnetic waves, light or sound. Regardless of the type of energy involved, every pulse transit time method measures PTT for its further conversion into a sought distance and suffers from passive and active disturbances. The passive disturbances result from reflections produced at fixed targets that differ from the true target. The active disturbances are created either by spurious sources of the same physical nature the transmitted pulse burst is or caused by electromagnetic interference at inputs of the measuring system's electronic hardware.
The U.S. Pat. No. 5,877,997 may serve as an example of using ultrasound pulses for the pulse echo distance measurement. The method of this patent implements the sought distance determination by measuring the pulse transit time of the main (first) transmitted pulse train and including the derived pulse echoes into the analysis to provide self-diagnostic checks and eliminate spurious information that might be produced by the system's elements internal delays. A particular advantage of this method is that a very small distance between the sensor and the target becomes measurable if the medium where the ultrasound pulses propagate is a fluid with low acoustic signal decrement. In those cases where the loss of the acoustic energy is high, such as in the air medium, the derived echoes are very weak limiting the applicability of the method of measurement. In addition, the method of the discussed patent does not address the problem of protecting the distance measurement against the passive and active disturbances.
Another example of the acoustic pulse transit time method for distance measurement is described in the U.S. Pat. No. 5,793,704 where issues relating to the transit time accurate measurement are well defined. The patent discloses that low energy echo-signals and fluctuation in the velocity of sound might cause errors in the pulse transit time measurement. The PTT is measured through the obtaining and consequently analyzing the echo-envelope. According to the patent, the echo-envelope is formed by an emitted acoustic pulse burst of the constant amplitude, frequency and off-duty factor. For the PTT measurement, the patent recommends using the envelope's maximum value as a characteristic parameter of the echo-envelope. In addition, the patent suggests working at an upper threshold of the system's linearity that can be obtained through a calibration procedure, which is incorporated into each measuring cycle. Therefore, the time position of the echo-envelope's maximum becomes a critical parameter that defines the sought distance. In other cases, such a characteristic parameter might be a pulse's zero crossing or another local element of the echo-envelope that is observable through monitoring of the measuring system's signals; is stationary and uniquely defined by the system's rigid organization. The discussed example of the prior art uses a single variable that has the property of being highly sensitive to the moment of time the echo pulse burst returns to the sensor. However, this method does not distinguish between the true target reflection and other fixed target reflections and/or effect of active disturbances that might occur in the measuring cycle.
The U.S. Pat. Nos. 6,122,602 and 5,822,275 illustrate the utilization of electromagnetic wave trains in the pulse transit time method for distance measurement. Particularly, in the U.S. Pat. No. 5,822,275, a sampling procedure is applied for storing data describing echo-envelopes; the data are collected separately for the actual target and for a variety of other fixed targets that cause errors during the distance measurement. In the '275 patent, the emitted pulses of each measuring cycle are identical and follow at a given transmitting frequency. According to the patent disclosure, the implementation of the method of this invention requires prior to the measurement cycle, information related to the fixed target echos is acquired and stored for use for suppressing the fixed target echoes. Apparently, the above-described approach to the distance measurement requires a prior knowledge of the fixed targets disposition and is subject to error each time when a new unaccounted fixed target appears. This limits the method's applicability and reduces its operational speed. Furthermore, the discussed prior art method does not provide for the elimination of active disturbances from the process of distance measurement. However, the important idea of this method is the creation of the set of several characteristic values derived from the echo-envelope; this set is associated with each target and is used in a measuring cycle for suppressing echoes from the fixed targets whereby improving the accuracy of the PTT and the distance measurement. Also, it is important also to realize that the above-described set of characteristic values conforms to the unchangeable structure of the measuring device.
Application of the light pulses for distance measurement is described in the U.S. Pat. No. 5,699,151. The patent presents a device that measures distance based on the pulse transit time method. The device functions in the environment where the interference light exists and therefore, creates a disturbing addition to the incident light that comes to the device's light receiver. The patent describes the way of reducing harmful effect that the interference light causes to the accuracy of distance measurement. The principle of operation is explained in the patent disclosure as follows: “. . . when interference light exists in incident light, the interference light has the property that, as compared with the reflection light reflected by an object of measurement, the intensity is greater and a fluctuation in the intensity is small, and that incident light exists even after the distance measurement time equivalent to the distance measurable range in which time data is measured. In order to exclude the interference light from the process of measurement, the device of this patent, is equipped with an interference light detection means, “. . . determining whether interference light exists in the incident light, based on the intensity of the incident light”. Because of such action, the invalid distance readings are excluded from the device's output. This device demonstrates the feasibility of the pulse transit time method with the light waveform agent. At the same time, according to the device's method of operation, the readings are protected from the occurrence of the interference light at the aperture of the light receiving element. Similar to other shown above prior art examples, this method uses at least one specific characteristic of the reflected pulses—the intensity of light measured at the opening of the light receiving unit. However, reflections from the fixed targets and light emissions from spurious sources together with fluctuation in the medium transparency might cause the device to yield an invalid data, or the opposite, to block the distance measurement during several measuring cycles. None of the discussed above methods and devices provide with an inclusive, oscillating agent-independent and effective solution to the elimination of the passive and active disturbances from the pulse transit time-based method for distance measurement.
Another measuring technique must be mentioned in the prior art section of the present invention. This technique suggests modulation of the transmitted pulses for increasing the signal-to-noise ratio during the distance measurement. The importance of the emitted pulses modulation is that such a technique aims directly against the possible disturbances of the measuring process. The U.S. Pat. Nos. 6,128,982 and 5,892,576 are indicative with respect to the transmitted pulses modulation in methods for distance measurement. The device of the U.S. Pat. No. 6,128,982 implements a two-step pulse transit time method where the first step is for the coarse determination of the PTT and respectively the sought distance; the second step is for the fine distance measurement through monitoring of the phase shift between the transmitted pulse burst and the returned echo-signal. During the first step of a measuring cycle, the emitted pulse of the electromagnetic wave is modulated by a pseudo-random digital code of finite length. Once received, the echo-signal is converted into a digital code for its further correlation analysis with the modulating pseudo-random code that was generated at the beginning of the measuring cycle. The purpose of the correlation analysis is a coarse determination of the pulse transit time. In general, the principle of the transmitted pulses modulation helps reduce the effect that active and/or passive disturbances might have upon the accuracy of the distance measurement. However, the pseudo-random code modulation might become harmful rather then beneficial with regards to the accuracy of distance measurement. The echo-envelope resulting from the above-mentioned type of modulation might obtain a shape similar to the one that occurs by virtue of the action of some passive and/or active disturbances. Then, the correlation between the pseudo-random code and the echo-envelope digital representation could be established at an incorrect instance of time leading to the incorrect calculation of PTT. In addition, due to the stochastic nature of the echo-envelope characteristic variable, which is the correlation function peak, the coarse PTT measurement mandates the second step of a measuring cycle: sending another pulse burst, receiving corresponding echo signal, and evaluating the phase difference between the echo signal and the delayed transmitted signal. The delayed transmitted signal is the transmitted pulse burst of the second step that was previously delayed by the value of the measured PTT of the first step. The process of the second step is not protected from the signal distortion that any passive and/or active disturbance might cause.
The method of the U.S. Pat. No. 5,892,576 uses modulation of transmitted pulses too. The oscillating agent of this method is a pulse burst of light provided by a laser and received as echoes by a receiver. According to the abstract of the patent, “(T)he pulses are each produced within respective intervals having a common predetermined duration. Each pulse is time-shifted relative to the beginning of its corresponding interval. A pulse pattern comprising echo signals for each of a series of pulses is used to determine the distance to an object producing the echoes”. The analysis of the last two prior art patents leads to the understanding that every pulse-code method for distance measurement utilizes the idea of reducing the probability of erroneous capturing the incoming echo-signal due to the creation of a specific echo-pattern through the code-modulation of the emitted pulses. A calculation of probabilities in the simple example below demonstrates the efficiency of this prior art approach. The probability of the event that a single echo-pulse taken by a receiver is an error is denoted by p. Considering the possible method's embodiment where the only occurrence of the three consequent pulses is the “true” echo, reduces the probability (p1) of receiving the “false” echo to the power of three, p1=p3, which is, for instance, 1000 times less then the probability p observed in conventional methods if given p=0.1. The authors of the last patent suggest the application of a variety of known statistical procedures for the comparison of the pattern of emitted pulses with the pattern of the received echo-pulses. For the direct pattern comparison, the reference pattern is obtained through a calibration measurement of “the distance of the reference measurement object from the measuring instrument being approximately zero”. Statistical methods are naturally time and memory consuming with limited accuracy. In addition, the method of the discussed invention offers only one specific type of modulation with the duration of the time-shift interval greater then the relaxation time of the laser. It is important to admit that the type of modulation of the last patent does not protect against the accuracy reducing effect from the variety of passive disturbances existing in the medium (reflections from fixed targets).
The analysis of the prior art solutions leads to a conclusion that for the PTT monitoring the following techniques or combination of them are used:    Scalar or vectorial characteristic elements of the received echo-signal; the echo-signal exists in two major forms: (a) the form of a pattern of separate pulses—the emitting element relaxes before generating each next pulse; (b) the form of an echo-envelope—the emitting element generates the next pulse being still involved in the process of emitting the previous one    Signal-to-Noise ratio improving modulation of the transmitted pulses including various versions of the pulse-code modulation
In addition, one common property must be attributed to all prior art pulse transit time-based methods for distance measurement: none of these methods adapt to the environment; the echo-developing technique and echo-processing algorithms of the prior art do not reflect changes in the membership of the sets of passive and/or active disturbances. However, the transience of the environment where the distance measurement is being performed could seriously reduce the accuracy of the PTT and consequently the distance measurement.
Therefore, the object of the present invention is the improvement of the accuracy and the operational speed of pulse transit time-based methods for distance measurement regardless of the transmitted pulses' physical origin; the distance measurement is being carried out under the intensive influence of multiple passive and active disturbances that occur in the chaotically changing environment.